Mullen said military planning was under way to overcome that risk. He said the increase of 21,000 US forces in Afghanistan was "about right" for the new strategy of trying to quell the insurgency and speed up training of Afghan security forces.

"Can I [be] 100% certain that won't destabilise Pakistan? I don't know the answer to that," Mullen told the committee.

He was responding to Senator Russ Feingold, a Democrat who said he was concerned the buildup might push militants into already troubled Pakistani regions and "end up further destabilising Pakistan without providing substantial, lasting" improvement in Afghanistan.

"I share your concern," Mullen said. "Your point about insurgents going particularly into Baluchistan, but particularly across that border ... we all share the concern for that," Mullen said.

"Where I'm comfortable is at least planning for it and having some expectation [that] will allow us to address that," Mullen said.

"Pakistan is further away from being totally destabilised than a lot of people realise."

He said he did not know of "any other way to provide for the security" needed in Afghanistan other than sending more troops.

The Pakistani military chief, General Ashfaq Kayani, has told US officials he is worried not only about Taliban forces moving across the border, but the possibility that US troops could prompt an exodus of refugees from southern Afghanistan.

Marine commandant General James Conway, whose forces are entering southern Afghanistan, told a Pentagon news conference last month of Kayani's concern but said no one knew for sure where insurgents would move as US operations increased.

"There are others that think they may go in different directions," Conway said. "But in any event, we've got to do what we've got to do in the south."